Thermodynamics of the N2/N3- redox couple in a LiBr-KBr-CsBr melt

Nitrogen electrode reaction has been investigated in a LiBr-KBr-CsBr melt containing Li₃N. The reaction N^3- → 1/2N₂ + 3e^- is confirmed by quantitative analysis of anodically evolved gas. The Nernst relation holds for the rest potential of Ni electrodes at a nitrogen gas pressure, p_N2, of 0.05-1.0 atm and an anion fraction of the N^3- ions, x_N3-, of 0.003-0.010 (anion fraction). Then, the standard formal potential of the N₂/N^3- couple, E_N2/N3-^′, is evaluated to be 0.251 ± 0.009 V versus Li⁺/Li (p_N2 = 1 atm, x_N3 = 1) at 673 K. The dependence of E_N2/N3-^′ on the temperature (570-730 K) gives a linear relation, whose slope is (-0.930 ± 0.117) × 10^-3 V K^-1. Thermodynamic quantities for the formation of Li₃N in the melt are also estimated.